A junction box cover assembly for mounting a fixture on a wall or ceiling. The assembly includes an adapter plate for being mounted on a junction box. A patch for supporting the cover assembly on the wall or ceiling in a mounted condition is provided. The patch is adapted to rest flush against the wall or ceiling and to be covered by and embedded in a joint compound. A hub having a body is insertable through openings of the patch and the adapter plate. The assembly includes elements for securing the hub to the adapter plate and elements for securing the hub to the patch. An insert is attachable to the hub for mounting a fixture or providing a blank-up cover.